How to Trace TCP Socket TCP_INFO tcpi_retrans_synack TCP Authentication Option Drops in Linux Using eBPF

In modern enterprise and telecom routing backbones running on Linux, the TCP Authentication Option (TCP-AO, RFC 5925) supersedes legacy MD5 protection for securing BGP and LDP sessions. When key ID mismatches, out-of-order master key tuples (MKT), or invalid MAC signatures occur during connection negotiation, the Linux kernel silently drops the incoming packet. Consequently, the server executes repetitive SYN-ACK retransmissions, tracked inside tcpi_retrans_synack. Extended Berkeley Packet Filter (eBPF) provides kernel-level visibility into these cryptographic verification drops without disrupting routing engines.

Mechanisms of TCP-AO Verification Drops

When TCP-AO is active on a socket, the kernel verifies the cryptographic MAC header on every segment via tcp_ao_inbound_verify(). If the key index does not match the active MKT or if authentication fails, the function returns a verification error, prompting the kernel to drop the segment without generating an ICMP unreachable reply. The server’s retransmission timer fires repeatedly, incrementing tcpi_retrans_synack until exhaustion. Tracing kernel TCP-AO routines exposes configuration discrepancies instantly.

Constructing the bpftrace TCP-AO Verification Monitor

The following bpftrace script captures failed TCP-AO hash evaluations and correlates them with SYN-ACK retransmissions:

#!/usr/bin/env bpftrace

#include <net/sock.h>
#include <net/inet_connection_sock.h>
#include <linux/tcp.h>

BEGIN
{
  printf("Tracing TCP-AO signature verification drops... Hit Ctrl-C to finish.
");
}

kprobe:tcp_ao_inbound_verify
{
  $sk = (struct sock *)arg0;
  $skb = (struct sk_buff *)arg1;

  @ao_checks_total = count();
}

kretprobe:tcp_ao_inbound_verify
{
  if (retval != 0) {
    printf("[%s] TCP-AO Signature Mismatch Detected (Verification Failure)
",
      strftime("%H:%M:%S", nsecs));
    @ao_mismatches = count();
  }
}

kprobe:inet_rtx_syn_ack
{
  $req = (struct request_sock *)arg1;
  $daddr = ntop($req->ir_rmt_addr);
  $retries = $req->num_retrans;

  @synack_retries_under_ao[$daddr] = count();
}

interval:s:15
{
  time("%H:%M:%S ");
  printf("--- TCP-AO Verification & Retransmission Summary ---
");
  print(@ao_mismatches);
  print(@synack_retries_under_ao);
  clear(@ao_mismatches);
  clear(@synack_retries_under_ao);
}

END
{
  clear(@ao_checks_total);
  clear(@ao_mismatches);
  clear(@synack_retries_under_ao);
}

Running and Interpreting Kernel Measurements

Execute the script on your target server using administrative root capabilities:

sudo bpftrace synack_ao_drops.bt

The diagnostic console outputs real-time TCP-AO drop notifications:

[16:58:10] TCP-AO Signature Mismatch Detected (Verification Failure)
16:58:25 --- TCP-AO Verification & Retransmission Summary ---
@ao_mismatches: 8
@synack_retries_under_ao[198.51.100.102]: 4

Remediation Strategies and Configuration Auditing

When recurrent TCP-AO signature handshake drops are identified:

• Verify MKT Key Identifiers: Confirm that Current Key ID and RNext Key ID parameters align identically across both peering configurations.
• Check Algorithm Selection: Verify that HMAC-SHA-1-96 or AES-128-CMAC-96 algorithms are consistently specified without protocol divergence.
• Audit Interface Binding: Ensure that TCP-AO socket filters are bound to the correct network namespaces and physical interfaces.